Catalyst for the synthesis of methyl mercaptan and process for producing methyl mercaptan from synthesis gas and hydrogen sulphide

ABSTRACT

The invention relates to a catalyst comprising an active component based on molybdenum and on potassium and a support based on hydroxyapatite, and also to a process for preparing said catalyst and a process for producing methyl mercaptan in a catalytic process by reaction of carbon monoxide, sulphur and/or hydrogen sulphide and hydrogen, comprising the use of said catalyst.

The work that led to this invention received financing from the EuropeanUnion as part of the 7th Framework Programme (FP7/2007-2013) underproject number No. 241718 EUROBIOREF.

The present invention relates to a specific molybdenum- andpotassium-based catalyst that is useful for producing methyl mercaptanfrom synthesis gas and hydrogen sulfide, and to its preparation process.

The invention also relates to a process for producing methyl mercaptanthat uses this catalyst.

The invention lastly relates to the use of hydroxyapatite as a supportfor a catalyst for producing methyl mercaptan.

Methyl mercaptan has great industrial interest, particularly as a rawmaterial for synthesizing methionine, an essential amino acid that is inwidespread use in animal food. Methyl mercaptan is also a raw materialfor many other molecules, in particular dimethyldisulfide (DMDS), asulfidation additive for hydrotreating catalysts in petroleum fractions,among other applications.

Methyl mercaptan is commonly produced in large tonnages industriallyfrom methanol and hydrogen sulfide. It may prove economicallyinteresting to want to produce methyl mercaptan directly from carbonmonoxide, hydrogen and hydrogen sulfide according to the followingreaction scheme:

CO+2H₂+H₂S→CH₃SH+H₂O  (1)

The main by-product from this synthesis is carbon dioxide. Carbonylsulfide (COS) is considered to be the reaction intermediate, which leadsto methyl mercaptan after hydrogenation according to the followingreaction schemes:

CO+H₂S→COS+H₂  (2)

COS+3H₂→CH₃SH+H₂O  (3)

The carbon dioxide comes from two side reactions:

CO+H₂O=CO₂+H₂  (4)

and

COS+H₂O→CO₂H₂S  (5)

These two side reactions, which consume the main raw material: carbonmonoxide, and the reaction intermediate: carbonyl sulfide, are due tothe inescapable presence of water, coproduced during methyl mercaptansynthesis. The carbon dioxide can optionally be recycled to producemethyl mercaptan as well according to the following scheme:

CO₂+3H₂+H₂S→CH₃SH+2H₂O  (6)

But this reaction is known to be slower than that from carbon monoxide.Therefore there is incentive to make carbon dioxide production as low aspossible at the outlet of the methyl mercaptan reactor.

From document WO2005/040082 several catalysts are known for thesynthesis of methyl mercaptan from synthesis gas and hydrogen sulfide.

In particular, this document discloses the use of a catalyst comprisinga Mo—O—K based active component, an active promoter and optionally asupport. The catalysts exemplified have different chemical natures, suchas K₂Mo₄/Fe₂O₃NiO or K₂MoO₄/CoO/CeO₂/SiO₂, each supported on silica.This leads to a CO₂/MeSH selectivity ratio of 0.88 at 333° C.

A family of catalysts composed of a porous support onto which a metalhas been deposited electrolytically is also known from documentUS2010/0286448. K₂MoO₄ and another metal oxide as promoter were thenimpregnated onto this support. Example 15 of this document describes thepreparation of K₂MoO₄/NiO/CoSiO₂. The CO₂/MeSH selectivity ratio withthis complex catalyst is 0.65.

Lastly, US document 2010/0094059 describes supported K₂MoO₄ basedcatalysts, where the porous support used alone or in mixtures is chosenfrom SiO₂, Al₂O₃, TiO₂, Al₂O₃/SiO₂, ZrO₂, zeolites or carbon-containingmaterials. Tellurium oxide (TeO₂) is used as promoter. The CO₂/MeSHselectivity ratios are comprised between 0.60 and 0.77 measured at 300°C.

From the teaching of these documents it has been observed that combiningcatalysts with specific structures, promoters and supports, each beingcarefully selected, means that interesting selectivity ratios can beachieved.

There is a current need for a catalyst that is simply synthesized andleads to very good selectivity. This technical problem has been resolvedby a molybdenum- and potassium-based catalyst supported byhydroxyapatite.

It has been observed that the catalyst according to the invention iseasier to prepare, given that the presence of a promoter is notindispensable. It is less costly than those disclosed in the previouslycited documents. And lastly, it leads to very good CO₂/MeSHselectivities.

The invention also relates to the preparation process for this catalyst.

The invention also relates to a process for producing methyl mercaptanfrom synthesis gas and hydrogen sulfide using the catalyst according tothe invention.

The invention also relates to the use of the catalyst as defined abovefor the synthesis of methyl mercaptan from synthesis gas and hydrogensulfide.

Lastly the invention relates to the use of hydroxyapatite as support forpreparing a catalyst for producing methyl mercaptan, and in particularin a catalytic process by reacting carbon oxide, sulfur and/or hydrogensulfide and hydrogen.

Other characteristics, features, subjects and benefits of the presentinvention will emerge even more clearly on reading the description andthe examples that follow.

Any range of values denoted by the expression “between a and b”represents the values ranging from more than a to less than b (i.e.limits a and b excluded), while any range of values denoted by theexpression “from a to b” means the values ranging from a to b (i.e.including the limits a and b).

Catalyst

The present invention relates to a catalyst.

This catalyst comprises a molybdenum- and potassium-based activecomponent and a hydroxyapatite-based support.

Active Component

The active component present in the catalyst according to the inventioncomprises molybdenum and potassium within a single component.

Preferably, the molybdenum- and potassium-based active component ischosen from compounds based on Mo—S—K, compounds based on Mo—O—K, andtheir mixtures.

The Mo—S—K based active component may be obtained by deposit andcalcination of K₂MoS₄ or (NH₄)₂ MoS₄ precursors with impregnated K₂CO₃added separately to the support.

The Mo—O—K based active component may be obtained by deposit andcalcination of K₂MoO₄ or (NH₄)₂ MoO₄ precursors with impregnated K₂CO₃added separately to the support.

it is also possible to use ammonium heptamolybdate (NH₄)₆Mo₇O₂₄.4H₂O asreagent, in the presence of a potassium salt such as for instancepotassium nitrate KNO₃, potassium carbonate K₂CO₃ or potassium hydroxideKOH.

These compounds are precursors of Mo—S—K and Mo—O—K based active phasesrespectively. The active phases are obtained after in situ precursorpretreatment, with for example a procedure consisting in a first step ofdrying in nitrogen at 250° C., followed by sulfidation with hydrogensulfide at the same temperature for 1 hour, then a step ofreduction/sulfidation with H₂/H₂S at 350° C. for 1 hour.

Support

The catalyst support according to the invention is hydroxyapatite havingformula Ca₁₀(PO₄)₆(OH)₂, advantageously a stoichiometric hydroxyapatite.

Preferably, hydroxyapatite that is useful according to the presentinvention has a Ca/P molar ratio ranging from 1.5 to 2.1, and morepreferably 1.67, corresponding to the expected value for stoichiometrichydroxyapatite.

Preferably, the weight ratio of the catalyst according to the inventionis:

K₂MoS₄/Ca₁₀(PO₄)₆(OH)₂=31.3/100

K₂MoO₄/Ca₁₀(PO₄)₆(OH)₂=50.7/100

The catalytic activity may be improved by using a support materialhaving a specific area greater than 25 m²/g.

Preferably, the hydroxyapatite supports according to the invention havea specific area of at least 40 m²/g, more specifically the specific arearanges from 40 m²/g to 300 m²/g and a Ca/P molar ratio of 1.67.

The structure of the support may be three dimensional, spherical,cylindrical, ring-shaped, star-shaped, granulates or any other threedimensional shape, or in the form of a powder, which can be pressed,extruded, granulated or in a three dimensional shape.

Preferably, the catalyst particles have uniform particle sizedistribution with diameter from 0.1 mm to 20.0 mm measured by sieveanalysis.

Promoter

Preferably, the catalyst according to the invention consists in amolybdenum- and potassium-based active component and ahydroxyapatite-based support.

However, it is possible to envisage the presence of a promoter known tothe person skilled in the art, such as tellurium oxide, nickel oxide oriron oxide.

Catalyst Preparation Process

The invention also relates to the preparation process for the catalystaccording to the invention. This process comprises the followingsuccessive steps:

-   -   preparing the precursor for the active phase    -   preparing the support, and    -   dry impregnating the support with the active phase precursor.

Preparing the Precursor for the Active Phase

1/Mo—O—K

1. The K₂MoO₄ salt is a commercial salt. To prepare the Mo—O—Kbased-catalyst, a fixed quantity of K₂MoO₄ is dissolved in a volume ofwater to obtain a solution with desired concentration, such as forexample a concentration ranging from 0.5-1.0 g/mL.

2. It is also possible to begin with separated molybdenum and potassiumsalts, i.e. that are not part of the same compound. For this synthesis,a molybdenum-based solution is prepared by adding ammoniumheptamolybdate in water to obtain a MoO₃ concentration ranging from 22to 33% by weight.

In parallel, a potassium-based solution is prepared by adding potassiumnitrate in water to obtain a K₂O concentration ranging from 31 to 43% byweight.

2/Mo—S—K

The K₂MoS₄ synthesis is generally done in two steps.

The first step involves preparing ammonium tetrathiomolybdate (ATTM);the second step is the synthesis of potassium tetrathiomolybdate(K₂MoS₄) from the salt prepared in the first step.

To prepare ATTM, hydrogen sulfide is left to bubble continuously in a25% aqueous ammonia solution, in which ammonium heptamolybdate (HMA) hasbeen dissolved. The solution temperature increases, indicating anexothermic reaction. The hydrogen sulfide bubbling is stopped when thetemperature falls (generally after one hour).

The solution then contains red crystals with green reflections, whichcorrespond to ammonium tetrathiomolybdate.

The second step consists in an ion exchange between ammonium ions in theammonium tetrathiomolybdate salt obtained and potassium ions, which comefrom a potassium hydroxide solution. The salts obtained are then storedunder vacuum. A quantity of potassium tetrathiomolybdate is dissolved inwater.

The potassium salt useful in the catalyst according to the presentinvention may come from the following compounds: potassium acetate(KAc), potassium oxalate (K₂C₂O₄), potassium hydroxide (KOH), potassiumcarbonate (K₂CO₃), potassium nitrate (KNO₃), and potassium bicarbonate(KHCO₃).

Support Preparation

The catalyst support, constituted of hydroxyapatite, is prepared by acoprecipitation method. An aqueous solution of calcium nitrate Ca(NO₃)₂was added dropwise to an ammonium hydrogenphosphate (NH₄)H₂PO₄ solutionwith stirring. The temperature is held at 100° C. and the pH is held at10 with addition of an ammonia solution (25%).

The resulting white precipitate is filtered, washed, dried at 80° C.overnight and calcinated at 400° C. The hydroxyapatite Ca₁₀(PO₄)₆(OH)₂was obtained with a Ca/P molar ratio of 1.67 corresponding to theexpected value for a stoichiometric hydroxyapatite.

Dry Impregnating the Support with the Active Phase Precursor

1/Mo—O—K

The dry impregnation method is used to prepare the catalyst. The K₂MoO₄solution is impregnated in one step on the support. When the solutionscontaining potassium and molybdenum are distinct, the impregnation isdone in 2 steps.

2/Mo—S—K

A potassium tetrathiomolybdate solution is then impregnated ontohydroxyapatite. The molybdate content in the catalyst depends on theK₂MoS₄ or K₂MoO₄ solubility and the support's porous volume.

The K₂MoS₄ solubility is between 0.25 g/mL and 0.50 g/mL (0.35 g/mL) andthe K₂MoO₄ solubility is between 0.50 g/mL and 1.50 g/mL (0.90 g/mL).The support's porous volume is between 0.8 mL/g and 2.2 mL/g.

Consequently, the volume of solution used is calculated to obtain thedesired weight ratio, and preferably the weight ratio as defined above.

After impregnation, the solid undergoes a maturation step for 2 hours,then oven drying at 80° C. for 24 hours, and calcination under gas flow(typically air) at 490° C. for 4 hours. If a second impregnation step isnecessary, the solid undergoes the maturation, drying and calcinationsteps again.

Production Process for Methyl Mercaptan

The invention relates to a production process for methyl mercaptan in acatalytic process by reacting carbon oxide, sulfur and/or hydrogensulfide and hydrogen, comprising the use of a catalyst as defined above.

The CO or CO₂/H₂S/H₂ molar ratios range from 1/1/0 to 1/8/8, or whensulfur is used to replace hydrogen sulfide, the molar ratios of CO orCO₂/H₂S/H₂/S reagents range from 1/1/0/1 to 1/8/8/8.

Preferably, the CO or CO₂/H₂S/H₂ molar ratios range from 1/2/1 to 1/4/4,when sulfur is used to replace hydrogen sulfide, the molar ratios of COor CO₂/H₂S/H₂/S reagents from 1/2/2/1 to 1/4/4/4.

These molar ratios take CO₂ into account. Therefore, they consider bothreaction scheme (1) and reaction scheme (6).

Preferably, the reaction may occur in fixed tubular, multitubular,catalytic wall micro-channel or fluid bed reactors.

The invention also relates to the use of the catalyst as defined abovefor the production of methyl mercaptan from synthesis gas and hydrogensulfide.

Lastly the invention relates to the use of hydroxyapatite as support forpreparing a catalyst for producing methyl mercaptan, and in particularin a catalytic process by reacting carbon oxide, sulfur and/or hydrogensulfide and hydrogen.

The present invention will now be described in the examples below, theseexamples being given only for illustration, and are of course notlimiting.

EXAMPLES Example 1

The catalyst according to the invention is prepared according to the dryimpregnation method, as defined above.

The resulting catalyst has the following characteristics:

TABLE 1 Elemental analysis of the catalyst Catalyst Chemical composition(% by weight) Mo K S N K₂MoS₄/Hap 9.9 8.1 13.3 <0.10

Example 2

The catalyst used is K₂MoO₄ on hydroxyapatite

Example 3

The catalyst tested is K₂MoO₄ on SiO₂

Example 4

The catalyst tested is K₂MoS₄ on Al₂O₃

Example 5

The catalyst tested is K₂MoO₄ on Al₂O₃.

Evaluating the Catalysts

The catalysts are evaluated in a reaction to produce methyl mercaptan ina fixed-bed reactor in the following conditions:

Temperature: 280° C.,

Pressure: 10 bars,

Composition of CO/H₂/H₂S=1/2/1 feed gas (v/v),

GHSV (Gas Hourly Space Velocity)=1333 h⁻¹

The reagents and products were analyzed in-line by gas chromatography.

Before the test, the catalysts were activated in situ with a firstprocedure consisting in a first step of drying in nitrogen at 250° C.,followed by sulfidation with hydrogen sulfide at the same temperaturefor 1 hour, then a step of reduction/sulfidation with H₂/H₂S at 350° C.for 1 hour.

The results are in table 2 below.

TABLE 2 Results of catalytic tests Molar selectivities (%) ExamplesCatalyst CH₃SH COS CO₂ CO₂/CH₃SH ratio 1 (inv) K₂MoS₄/Hap 44.1 23.3 32.60.74 2 (inv) K₂MoO₄/Hap 43.3 23.6 31.9 0.74 3 (comp) K₂MoO₄/SiO₂ 48.85.3 45.3 0.93 4 (comp) K₂MoS₄/Al₂O₃ 45.0 7.3 46.6 1.04 5 (comp)K₂MoO₄/Al₂O₃ 47.0 3.4 49.6 1.06

The results presented in table 2 show that the catalysts according tothe invention (examples 1 and 2) give much lower CO₂ (undesired product)selectivities than catalysts on the supports in the prior art (silica:example 3 or alumina: examples 4 and 5).

The selectivities are compared using carbon monoxide isoconversion,where this conversion is expressed by m² of specific air in thecatalyst.

By comparing the results obtained with catalysts 1 and 4, we observe a30% improvement in ratio, and this improvement is linked to choosinghydroxyapatite as support.

The same observation is seen when comparing example 2 according to theinvention and examples 3 and 5.

We observe increased methyl mercaptan selectivity compared to the carbondioxide produced according to a side reaction.

It should be noted that this selectivity is obtained without aid fromthe promoter such as tellurium oxide, nickel oxide or iron oxide asdescribed in the prior art.

1. A catalyst comprising a molybdenum- and potassium-based activecomponent and a hydroxyapatite-based support.
 2. The catalyst as claimedin claim 1, wherein the catalyst support is hydroxyapatite havingstoichiometric formula Ca₁₀(PO₄)₆(OH)₂.
 3. The catalyst as claimed inclaim 1, wherein the molybdenum- and potassium-based active component isselected from the group consisting of compounds based on Mo—S—K,compounds based on Mo—O—K, and their mixtures.
 4. The catalyst asclaimed in claim 3, wherein the molybdenum- and potassium-based activecomponent has been obtained from a precursor having structure K₂MoS₄. 5.The catalyst as claimed in claim 4, wherein the weight ratio of K₂MoS₄and Ca₁₀(PO₄)₆(OH)₂ used to obtain the catalyst isK₂MoS₄/Ca₁₀(PO₄)₆(OH)₂=31.3/100
 6. The catalyst as claimed in claim 3,wherein the molybdenum- and potassium-based active component has beenobtained from a precursor having structure K₂MoO₄.
 7. The catalyst asclaimed in claim 6, wherein the weight ratio of K₂MoO₄ andCa₁₀(PO₄)₆(OH)₂ used to obtain the catalyst is:K₂MoO₄/Ca₁₀(PO₄)₆(OH)₂=50.7/100
 8. A preparation process for thecatalyst as defined in claim 1, comprising the following steps:preparing a precursor for the molybdenum- and potassium-based activecomponent; preparing the hydroxyapatite-based support; and dryimpregnating the hydroxyapatite-based support with the precursor for themolybdenum- and potassium-based active component.
 9. A process forproducing methyl mercaptan in a catalytic process by reacting carbonoxide, sulfur and/or hydrogen sulfide and hydrogen, comprising using acatalyst as defined in claim
 1. 10. A method for preparing a catalystuseful for producing methyl mercaptan, comprising using hydroxyapatiteas a support for preparing the catalyst.
 11. The catalyst as claimed inclaim 1, wherein the hydroxyapatite of the hydroxyapatite-based supporthas a Ca/P molar ratio ranging from 1.5 to 2.1.
 12. The catalyst asclaimed in claim 1, wherein the hydroxyapatite of thehydroxyapatite-based support has a Ca/P molar ratio of 1.67.
 13. Thecatalyst as claimed in claim 1, wherein the hydroxyapatite-based supporthas a specific area greater than 25 m²/g.
 14. The catalyst as claimed inclaim 1, wherein the hydroxyapatite-based support has a specific areagreater than 40 m²/g.
 15. The catalyst as claimed in claim 1,additionally comprising a promoter.
 16. The catalyst as claimed in claim15, wherein the promoter is selected from the group consisting oftellurium oxide, nickel oxide and iron oxide.